1. Field of the Invention
The present invention relates to an elastic-wave filter device, and, particularly, an elastic-wave filter device that includes a longitudinally coupled resonator-type elastic-wave filter section which has a balanced-to-unbalanced conversion function.
2. Description of the Related Art
Recently, elastic-wave filter devices that utilize an elastic wave such as a surface acoustic wave or a boundary acoustic wave at an RF stage of mobile phones have been used. The elastic-wave filter devices have been required to have not only a low insertion loss in the passband, but also excellent isolation characteristics in terms of electrical characteristics, a balanced-to-unbalanced conversion function in terms of a function, and so forth.
For example, in Japanese Unexamined Patent Application Publication No. 2010-109694, an elastic-wave duplexer in which a reception filter section is constituted by a longitudinally coupled resonator-type elastic-wave filter section that has the balanced-to-unbalanced conversion function is described. In the elastic-wave duplexer described in Japanese Unexamined Patent Application Publication No. 2010-109694, an elastic-wave resonator constituting a shunt trap is connected between a node between a reception filter section, which is constituted by a longitudinally coupled resonator-type elastic-wave filter section, and a first balanced-signal terminal and a node between the reception filter section and a second balanced-signal terminal.
However, the elastic-wave duplexer described in Japanese Unexamined Patent Application Publication No. 2010-109694 has a problem that it is difficult to sufficiently improve the common-mode isolation characteristics.